1. Field of the Invention
This invention relates to an improved well safety valve for use in controlling well fluids conducted through the well tubing bore. More particularly, the safety valve is adapted to be set at depths much deeper than previously possible, using a single control line for operation of the valve.
2. The Prior Art
Surface controlled subsurface safety valves have been used to control flow in a well at subsurface locations for some time. It is well known to provide a well safety valve to control the fluid flow in a well conduit by means of a valve closure member, disposed in the bore of the safety valve, which is movable between an open and a closed position. The closure member is moved to the bore open position by means of a piston and pressure chamber arrangement whereby control fluid is usually pressurized at the surface of the well and the fluid, conducted to the safety valve pressure chamber via suitable conduit, causes the piston to operate an operator member acting on the closure member. Release of pressure allows a spring or other resilient urging means to return the closure member to the bore closed position.
In order for the spring to push the operator member to its normal "at rest" position, closing the closure member, the spring must overcome the hydrostatic pressure exerted on the piston by the column of control fluid extending from the safety valve to the surface of the well. At deep well locations, this is an acute problem and thus, it is often decided to use a two-line, "balanced" safety valve, wherein the hydrostatic pressure in the control line is offset by the hydrostatic pressure of a second fluid filled conduit, extending from the safety valve to the surface of the well. Such balanced safety valves are well known. Reference is made to copending application Ser. No. 960,168 now U.S. Pat. No. 4193450, issued 3/18/80, filed by one of the present inventors, directed to a two-line balanced subsurface safety valve.
If, however, it were possible to strengthen the spring which closes the valve while reducing the annular area of the piston, single line subsurface safety valves could be set at much deeper locations. The limiting factor for selecting spring size and strength has always been the volume provided for housing the spring, in the safety valve. With the control piston located above the spring the safety valve housing must withstand the large pressure differentials due to placement of external seals; consequently, little internal spring housing volume remains.
Typically, with the piston and pressure chamber being located at an opposite end of the safety valve from the valve closure member, the closure spring has been placed therebetween. Such an arrangement provides only limited space for increasing spring size.
Consequently, an object of the present invention is to provide a surface controlled, subsurface safety valve, having a single control line, that can be set at depths in a well which are greater than previously practical for single control line safety valves.
Another object of the invention is to provide a surface controlled subsurface safety valve having a relatively large closure spring in relation to the piston surface area.
Yet another object is the provision of a very high piston pressure, in relation to the overall size of the safety valve of the present invention.
A further object of the present invention is to provide a surface controlled, subsurface safety valve having control fluid expandable pressure chamber-piston zone interspersed between the valve closure member and the closure spring housing.
Yet another object is to provide a means for equalizing the pressure between the bore of the safety valve of the invention, the interior closure spring housing, and the exterior of the closure spring housing above the packing in a retrievable valve.